JP3537686B2 - Information recording medium and its recording / reproducing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an information recording medium having a multilayer hologram formed by laminating a plurality of optically reproducible recording layers, and a method for recording and reproducing the same.

[0002]

2. Description of the Related Art Conventionally, there has been known an information recording medium on which a hologram capable of optically reproducing information is formed (for example, see Japanese Patent Application Laid-Open No. 4-56886).

[0003]

However, in the conventional information recording medium, only 1-bit information can be written in a unit recording area of one recording layer, and multi-valued information cannot be written. There was a problem. The present invention has been made to solve the above problems, and has as its object to provide an information recording medium capable of recording multi-valued information in a unit recording area and a recording and reproducing method thereof.

[0004]

According to the present invention, as described in claim 1, light incident from an end is propagated in a horizontal direction to generate a diffracted light from a concavo-convex pattern formed on an interface with a clad. recording layer to the (3) laminating a plurality of diffracted light 2
An information recording medium (1) capable of optically reproducing information by detecting with a two- dimensional photodetector has the above-mentioned concave and convex pattern for each unit recording area (4), which is a unit of recording information in each recording layer, A plurality of types of concavo-convex patterns represent multivalued information. As described above, by expressing multi-value information with a plurality of types of uneven patterns, multi-value information can be recorded in a unit recording area. In addition, as described in claim 2, the concave / convex pattern is defined in advance by a difference between the length and the number of convex portions or concave portions formed on the boundary surface between the recording layer and the clad. In addition, as described in claim 3, the uneven pattern is formed in a point-symmetrical shape with respect to the center of the unit recording area.
Further, as described in claim 4, the uneven pattern is
The height of the convex portion formed on the boundary surface between the recording layer and the clad or the depth of the concave portion is 0.05 μm to 0.2 μm.

According to a fifth aspect of the present invention, there is provided a recording layer which propagates light incident from an end in a horizontal direction and generates diffracted light from an uneven pattern formed on an interface with a clad. A recording / reproducing method for recording / reproducing information on / from an information recording medium capable of optically reproducing information by laminating a plurality of layers and detecting diffracted light with a two-dimensional photodetector. The multi-level information is recorded in each unit recording area of each recording layer by forming the above-mentioned uneven pattern in each unit recording area which is a recording unit of information in a layer and expressing multi-value information by a plurality of types of uneven patterns. It is intended to be. Further, as described in claim 6, the uneven pattern is:
It is defined in advance by the difference between the length and the number of the convex portions or concave portions formed on the boundary surface between the recording layer and the clad. Further, as described in claim 7, the uneven pattern is formed in a point-symmetrical shape with respect to the center of the unit recording area. Further, as described in claim 8, the height of the convex portion or the depth of the concave portion formed on the interface between the recording layer and the clad is 0.05 or less.
μm to 0.2 μm.

[0006]

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS [First Embodiment] Next, an embodiment of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a block diagram of an information reading device according to a first embodiment of the present invention. The information reading apparatus according to the present embodiment includes a light source (not shown) that emits a laser beam 11, an objective lens 12 that focuses the laser beam 11 on the memory card 1, and a focusing optic that focuses the light from the memory card 1. System 13 and a C that detects light that has passed through the condensing optical system 13 and converts the light into an electric signal.
A two-dimensional photodetector 14 comprising an area sensor such as a CD.

A memory card (information recording medium) 1 has a multilayer hologram 2 from which information can be optically reproduced. In the multilayer hologram 2 which is a large-capacity read-only memory, the recording layer 3 has a plurality of recording layers 3 (for example, 10 to 500) in the thickness direction.
Layers). At least one side wall of each recording layer 3 is formed so as to have a slope of about 45 degrees as shown in FIG. 1 in order to make the reproduction light enter the recording layer 3 horizontally.

[0009] On the surface of each recording layer 3, a concavo-convex pattern representing recording data calculated using Fourier transform optics in consideration of the light detection position is formed. Each recording layer 3 can be mass-produced using a transfer type or the like. By stacking a plurality of such recording layers 3, a large amount of data can be accumulated. The information reading device irradiates the memory card 1 with the laser beam 11 and detects leaked light from the multilayer hologram 2 to
Information is read from a desired recording layer 3 of the multilayer hologram 2.

FIG. 2A is a plan view of a unit recording area which is a unit of recording information in each recording layer 3, and FIG.
It is an AA line sectional view of (a). Each recording layer 3 is a planar waveguide (core) having a thickness of about 2 μm for transmitting light in the horizontal direction. As the material of the recording layer 3, polycarbonate (Poly Carbonate; PC) or polymethyl methacrylate (Poly Methyl Methacrylate; PMMA) is preferable. The recording layer 3 may be used alone or in combination of these materials.
To form

Each recording layer 3 is sandwiched from above and below by a cladding (not shown) having a lower refractive index than the recording layer 3 and having a thickness of about 10 μm. The amount of light diffracted can be controlled by the material (particularly the refractive index) of the recording layer 3 and the cladding. At this time, it is a good example that the diffraction efficiency is set to be 0.02 to 0.5%.

The unit recording area 4 (4-1 to 4-4) of each recording layer 3
-2m) has a size of about 0.3 to 1 μm. The present invention is characterized in that multi-value information is recorded in the unit recording area 4 of each recording layer 3. In order to record m-bit information in the unit recording area 4, 2 ^m types of concave and convex patterns may be prepared as shown in FIG. Accordingly, the intensity of light leaking from the concavo-convex pattern through the inside of the recording layer 3 can take a value of 2 ^m gradation, and thus m-bit information can be expressed.

The protrusions 5 formed on the surface of the recording layer 3 are formed along a direction perpendicular to the propagation direction of light propagating in the recording layer 3 (the left-right direction in FIG. 2). By changing the length and the number of the protrusions 5 in the orthogonal direction, 2 ^m kinds of uneven patterns can be defined. In the example of FIG. 2A, the unit recording area 4 where no concave / convex pattern is formed
No light leaks from -1, and the intensity of light leaking from the unit recording area gradually increases as the total area of the protrusions 5 increases, and the light intensity becomes maximum in the unit recording area 4-2m. .

When the wavelength of the laser beam 11 is 500 nm, the height of the convex portion 5 and the width in the light propagation direction are 0.05-0.
A good example is about 2 μm. Thereby, the efficiency with which light leaks from the concavo-convex pattern can be increased.
In the present embodiment, the projections 5 are formed on the surface of the recording layer 3 to form the concavo-convex pattern. However, the depressions may be formed on the surface of the recording layer 3. Further, in the present embodiment, an example has been described in which light leaking from the recording layer 3 is detected above the card 1, but a two-dimensional photodetector is provided on the opposite side of the card 1 or provided above and below the card 1. Or you may.

In order to reproduce the information recorded on the multilayer hologram 2 as described above, it is important to accurately guide light to each recording layer 3. For this purpose, first, the light source and the objective lens 12 are moved to the end of the multilayer hologram 2. Then, the laser beam 11 condensed by the objective lens 12 is applied to the inclined side wall of the desired recording layer 3 among the plurality of recording layers 3. Thereby, the laser beam 11 is refracted on the inclined side wall of the recording layer 3 to be reproduced, and propagates horizontally in the recording layer 3.

In the recording layer 3, the light propagates and the light leaks out from an uneven pattern formed on the surface of the recording layer 3. This leaked light is collected by a condensing optical system 13 installed on the multilayer hologram 2, and the unit recording areas 4 and 2 of the recording layer 3 are collected.
Optical magnification is performed so that each detection element (pixel) of the one-dimensional photodetector 14 corresponds one-to-one.

As described above, the unit recording area 4 of the recording layer 3
Is about 0.3 to 1 μm, and the size of one side of each detection element of the two-dimensional photodetector 14 is 5 to 10 μm.
It is about μm. Therefore, in the condensing optical system 14, optical magnification of about 5 to 10 times is performed. Thus, the two-dimensional photodetector 14 can detect the leak light from each unit recording area 4 of the recording layer 3.

The condensing optical system 13 and the two-dimensional photodetector 1
The recording layer 3 of the multilayer hologram 2 can be selectively reproduced by integrally moving the recording layer 3 up and down. Although a circular lens is used for the objective lens 12 in FIG. 1, a semi-cylindrical cylindrical lens may be arranged along the depth direction of FIG. 1 and used as the objective lens. Further, providing an image rotation optical system using a Dove prism or the like in front of the objective lens is also a good example for adjusting the incident position of the cylindrical lens.

FIG. 3 is a diagram showing a light intensity distribution obtained on the two-dimensional photodetector 14. In FIG. 3, each square indicates each detection element of the two-dimensional photodetector 14. Due to the optical expansion by the condensing optical system 13, light leaked from each unit recording area 4 of the recording layer 3 enters a corresponding detection element. The example of FIG. 3 shows an example in which three levels of light intensity of large (white portion in FIG. 3), middle (shaded portion), and small (black portion) are obtained.

Next, a description will be given of a reproduction method for reproducing information by converting light detected by the two-dimensional photodetector 14 into a reproduction signal. FIG. 4 is a flowchart for explaining a method of reproducing information. First, each detection element of the two-dimensional photodetector 14 converts a detected optical signal into an electric signal (step 101). By each detector performing photoelectric conversion,
A two-dimensional output signal f (x,
y) is output.

[0021]

[0022]

[0023]

Next, an equalization processing means (not shown) performs an equalization processing for removing the influence of interference between the unit recording areas included in the two- dimensional output signal f (x, y) (step 10).
3). The reason for performing the equalization process is that each detection element of the two-dimensional photodetector 14 detects not only the leaked light from the corresponding unit recording area but also the leaked light from the surrounding unit recording area. It is.

In order to perform this equalization processing, one unit recording area 4 whose signal amplitude is known in advance is read and
An interference transfer function B (x, y) indicating the relationship between the signal amplitude of the detection element corresponding to the unit recording area 4 and the signal amplitudes of the detection elements around the detection element is obtained. Subsequently , the two- dimensional output signal f
Inverse function B of (x, y) and interference transfer function B (x, y)
^-1 (x, y) and multiplying the two-dimensional output signal C (x, y)
y). C (x, y) = f (x, y) × B ⁻¹ (x, y) (2)

Note that this equalization processing is performed by the output signal f (x,
y) may be performed collectively, or the output signal
For each element of f (x, y), only the surrounding elements of 3 × 3 pixels or 5 × 5 pixels may be subjected to the interference transfer function B (x, y) and processed for each element.

Next, detection means (not shown) performs detection processing for generating a final reproduced signal from the two-dimensional output signal C (x, y) subjected to the equalization processing (step 104). In the detection process, the best signal obtained at the time of normal reproduction is prepared in advance as an ideal signal.
This is performed by comparing (x, y) with the ideal signal closest to the output signal C (x, y) as the reproduction signal.

In order to select the ideal signal closest to the output signal C (x, y), the difference between the signal amplitudes of the output signal C (x, y) and the ideal signal is calculated for each element, and each is calculated as 2 , And add these to obtain a square error. The calculation of such an error is performed for each ideal signal, and the ideal signal with the smallest error is selected as a reproduction signal.

As a method of generating a reproduction signal,
A method performed for each element of the output signal C (x, y), and 2 to 10
There are two methods: one for some elements and the other for all elements. When a reproduced signal is generated for each element of the output signal C (x, y), an ideal signal of at most m bits may be prepared. When a reproduced signal is generated for each of a plurality of elements of the output signal C (x, y) or collectively for the entire element, an ideal signal pattern corresponding to the reproduced signal may be prepared.

Further, in the present embodiment, the square error is used in obtaining the maximum likelihood (probability). However, the difference between the signal amplitude of the output signal C (x, y) and the ideal signal is used. An absolute value may be calculated for each element, and a value obtained by adding these values may be used. The error may be obtained by nonlinear addition of the original value using a sigmoid function or the like. As described above, information of a desired recording layer 3 of the multilayer hologram 2 can be reproduced.

[0031] In the present embodiment, an equal process (step 103), the detection process may be repeated two processes (step 104). By repeating the processing, an image having a distorted result in a certain process can be set to an optimum value.

Further, when the process is repeated, the correction amount is multiplied by a coefficient (0.1 to 0.5) to perform the correction gradually, thereby suppressing a large error from entering at the correction stage. can do. However, there is no correction amount in the detection processing. Moreover, at equal treatment, interfering the transfer function B (x, y) signal amplitude around the sensing device of the corresponds to the correction amount.

[Second Embodiment] FIG. 5 is a plan view of a unit recording area in each recording layer of a multilayer hologram according to a second embodiment of the present invention. In the first embodiment, the concave / convex pattern is formed asymmetrically in each unit recording area 4. On the other hand, in the present embodiment, the concavo-convex pattern is formed in a point-symmetrical shape with respect to the center of the unit recording area 4 (4-1 to 4-2m). Thus, the distribution of the leak light generated from each unit recording area 4 can be made uniform, and the leak light can be reliably reproduced by the two-dimensional photodetector 14.

[Embodiment 3] FIG. 6 is a flowchart for explaining a method of reproducing information according to a third embodiment of the present invention. In FIG. 6, the same processes as those in FIG. It is attached. In the present embodiment, as described above, the leakage light from the first recording layer to be reproduced is detected by the two-dimensional photodetector 14 (step 101), and the second light immediately above the first recording layer is detected. Laser light 11 is incident on the recording layer of
Leakage light from the second recording layer is detected by the two-dimensional photodetector 14 (step 105), and the laser beam 11 is made incident on the third recording layer immediately below the first recording layer, thereby obtaining the third light. Is detected by the two-dimensional photodetector 14 (step 106). These steps 101, 105,
The order of detection by 106 may be changed.

Subsequently, the correction means (not shown) multiplies the output signal of each detection element obtained by the second recording layer by a negative conversion coefficient c1 and simultaneously outputs each detection element obtained by the third recording layer. Is multiplied by the negative conversion coefficient c2, and the output signals of the respective detection elements obtained by the first, second, and third recording layers are added (step 107).

Thus, the influence of interference between the recording layers can be suppressed. Subsequent processing is the same as in the first embodiment. The conversion coefficients c1, c2 in advance signal reads the specific information amplitude has been found to perform equal treatment and detection processing, may be obtained an optimum value based on the result.

[0037]

According to the present invention, multivalued information can be recorded in a unit recording area by expressing multivalued information with a plurality of types of concave and convex patterns. As a result, the recording density of the information recording medium can be improved.

Further, by forming the concavo-convex pattern in a point-symmetrical shape with respect to the center of the unit recording area, the distribution of leaked light generated from each unit recording area can be made uniform. And the leaked light can be reliably reproduced by the two-dimensional photodetector.

According to a fourth aspect of the present invention, the height of the convex portion or the depth of the concave portion formed on the surface of the recording layer is set to 0.05 μm to 0.2 μm, so that the uneven pattern can be obtained. The efficiency with which light leaks can be increased.

Further, as described in the ninth aspect, the information on the first recording layer can be reproduced by detecting the difference in the amount of light from each unit recording area with a two-dimensional photodetector. Further , by correcting the output signal of each detection element based on the output signals of the surrounding detection elements, it is possible to suppress the influence of interference between unit recording areas.

According to a tenth aspect of the present invention, the output signal of each detection element obtained by the first recording layer is converted to a second,
By performing correction based on the output signals of the respective detection elements obtained by the third recording layer, it is possible to suppress the influence of interference between the recording layers.

[Brief description of the drawings]

FIG. 1 is a block diagram of an information reading device according to a first embodiment of the present invention.

FIG. 2 is a plan view and a sectional view of a unit recording area in each recording layer of the multilayer hologram of FIG.

FIG. 3 is a diagram showing a light intensity distribution obtained on a two-dimensional photodetector.

FIG. 4 is a flowchart for explaining a method of reproducing information.

FIG. 5 is a plan view of a unit recording area in each recording layer of a multilayer hologram according to a second embodiment of the present invention.

FIG. 6 is a flowchart for explaining a method of reproducing information according to a third embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Memory card, 2 ... Multilayer hologram, 3 ... Recording layer, 4 ... Unit recording area, 5 ... Protrusion, 11 ... Laser light, 1
2: Objective lens, 13: Condensing optical system, 14: Two-dimensional photodetector.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-197969 (JP, A) JP-A-9-101735 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G11B 7/24

Claims (8)

(57) [Claims] 1. A recording layer to cause diffracted light of light incident from the end of the concavo-convex pattern formed on the boundary surface between the clad is propagated horizontally stacking a plurality, the diffracted light 2
In an information recording medium capable of optically reproducing information by detecting with a three- dimensional photodetector, the information recording medium has the concavo-convex pattern for each unit recording area which is a unit for recording information in each recording layer, and includes a plurality of types of concavo-convex patterns An information recording medium representing multi-valued information. 2. The information recording medium according to claim 1, wherein the concavo-convex pattern is defined in advance by a difference in length and number of protrusions or recesses formed on a boundary surface between the recording layer and the clad. Characteristic information recording medium. 3. The information recording medium according to claim 1, wherein the concavo-convex pattern is formed in a point-symmetrical shape with respect to a center of the unit recording area. 4. The information recording medium according to claim 1, wherein the concave / convex pattern has a height of a convex portion or a depth of a concave portion formed on a boundary surface between the recording layer and the cladding of 0.05 μm.
An information recording medium characterized by having a thickness of about 0.2 μm. 5. A recording layer to cause diffracted light of light incident from the end of the concavo-convex pattern formed on the boundary surface between the clad is propagated horizontally stacking a plurality, the diffracted light 2
A recording / reproducing method for recording / reproducing information on / from an information recording medium capable of optically reproducing information by detecting with a two- dimensional photodetector , wherein each unit recording area is a recording unit of information in each recording layer. Recording / reproducing an information recording medium, wherein the multi-level information is recorded for each unit recording area of each recording layer by forming the concavo-convex pattern on the recording medium and expressing multi-level information by a plurality of types of concavo-convex patterns. Method. 6. The recording / reproducing method for an information recording medium according to claim 5, wherein the concave / convex pattern is defined in advance by a difference in length and number of convex portions or concave portions formed on a boundary surface between the recording layer and the clad. Recording / reproducing method for an information recording medium. 7. The recording / reproducing method for an information recording medium according to claim 5, wherein the concave / convex pattern is formed in a point-symmetrical shape with respect to a center of the unit recording area. Recording and playback method. 8. The recording / reproducing method for an information recording medium according to claim 5, wherein the concave / convex pattern has a height of a convex portion or a depth of a concave portion formed at a boundary surface between a recording layer and a clad of 0.05 μm.
Method of recording and reproducing an information recording medium which is a ～0.2Myuemu.